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The Week In Technology Graham Warwick Solid-State Propulsion First for MIT Dispensing with propellers and turbines, the first flights of an aircraft with solid-state propulsion have been conducted by researchers at the Massachusetts Institute of Technology (MIT). The lightweight unmanned aircraft made 10 sustained level flights indoors, using electroaerodynamic (EAD) propulsion. Launched by bungee into steady level flight, the 5-m-span (16.4-ft.), 2.45-kg (5.4-lb.) aircraft was able to fly at just under 5 m/s (11 mph) for 50 m across a gym at MIT on 3.2 newtons (0.7 lb.) of thrust produced by the 500-watt EAD, or ionic wind, propulsion system. The system comprises a thin filament, called the emitter, that is charged to 20,000 volts, creating an extremely strong electric field that removes electrons from nitrogen in the air flowing over the wire. These charged ions are drawn downstream to a thin airfoil, or collector, charged to -20,000 volts. As they flow from emitter to collector, the ions collide with neutral air molecules, transferring energy to the airflow and generating an ionic wind that is used to propel the aircraft. The propulsion system on MIT’s UAV comprises eight emitter-collector pairs strung along the span under the wing. How far the system can be scaled from this initial demonstration has not been determined. But the research team believes that solid-state propulsion technology could be applied to near-silent unmanned aircraft, high-altitude pseudo-satellites that would stay aloft indefinitely without requiring maintenance, and extremely small, swarming air vehicles that are too tiny for conventional propulsion.

Time-lapse image of MIT’s UAV sustaining steady level flight on solid-state “ionic wind” propulsion. Credit: Massachusetts Institute of Technology

Ionic wind propulsion was first investigated in the 1920s, and again in the 1960s, and dismissed as unworkable for aircraft, says Steven Barrett, an associate professor in MIT’s Department of Aeronautics and Astronautics. The breakthrough follows five years of fundamental research to understand how to produce efficient EAD propulsion and builds on advances in power electronics, he says. Power electronics that can convert battery power from 100 volts to 40,000 volts previously were not possible and are part of broader revolution in the electrification of all types of transportation, says David Perreault, a professor in MIT’s Department of Electrical Engineering and Computer Science. To meet the requirement for very low weight, the EAD-powered UAV has a custom battery and a high-voltage power converter with a specific power 5-10 times higher than equivalent conventional power supplies. This is enabled by operating at a much higher switching frequency to minimize weight, he says. The next steps in MIT’s research, Barrett says, are to look at different ways of ionizing the air and to use ionic wind for flight control as well as propulsion to eliminate control surfaces. The researchers also want to create an aircraft skin that is propulsive, eliminating the draggy array of filaments.

Electroaerodynamic thruster comprises eight pairs of emitter filaments and collector airfoils strung under the wing. Credit: Massachusetts Institute of Technology

“Rather than a separate propulsion system producing thrust and a fuselage generating drag, we want to integrate the two and have a skin that produces thrust instead of drag, creating a slipstream-propelled vehicle,” Barrett says. “The propulsion system would not be visible, and the aircraft would be silent.” EAD propulsion is throttleable over some range of voltages, but Barrett envisages a system that is dynamically reconfigurable between high thrust but low efficiency for takeoff and low thrust but high efficiency for cruise. “That is something we would like to look at,” he says. EAD propulsion does not produce the combustion emissions that are an issue with turbine engines, but the production of ozone as a result of the high-voltage emitter’s corona discharge is a concern. “We are starting work to measure the production rate and do some engineering to limit it,” says Barrett. “It will take several years of work to understand that side of the problem.” Noting that “turbines and propellers are not intrinsically safe” but aviation knows how to manage the risks, Perrault argues that the potential safety hazards posed by EAD propulsion’s high voltages “can be managed in an analogous way.” Thunderstorms, precipitation and turbulence are not expected to disrupt ionic-wind propulsion, and the system may offer some advantages, the researchers say. Barrett sees the technology potentially scaling in two directions. One is to extremely small vehicles, as “solid-state tends to scale down,” he says. The other is to larger and faster aircraft. This would enable more efficient EAD propulsion than in the small, low-speed test UAV, which achieved an efficiency of barely 2.6%. “There could be a maximum useful size, but we don’t know yet if there is a limit,” he says. Having scaled up from experiments 3-4-cm (1-1.5-in.) across to powering a 5-m-span UAV, Barrett does not see a challenge in scaling the technology up by another factor of 10. “What is potentially limiting is the density of the propulsive force as the aircraft gets heavier. At some point, the size of the propulsion system will get bigger than the size of the aircraft, and you will not end up with a sensible design.” EAD could be used in larger aircraft as part of a hybrid-electric propulsion system, with turbines or solar arrays providing the electrical power, Barrett says. Another potential use of the technology is to fill in flow distortions and reduce the noise caused by turbulence over the airframe. “By manipulating an electric field, we could eliminate airframe noise,” he says.